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Risk Management Activities - Effect of Hedges (Details) MWh in Millions, $ in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 CAD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2020 USD ($) MWh | Dec. 31, 2020 CAD ($) MWh | Dec. 31, 2019 USD ($) MWh | Dec. 31, 2019 CAD ($) MWh
Disclosure of detailed information about hedging instruments [line items] |  |  |  |  |  |
Hedging instrument, assets |  |  |  | $ 2 |  | $ 7
Physical power sales | Cash flow hedges | Commodity risk management |  |  |  |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |  |  |  |
Notional amount purchased | MWh |  |  | 16 | 16 | 19 | 19
Hedging instrument, assets |  |  |  | $ 573 |  | $ 678
Gain (loss) on hedge ineffectiveness | $ (33) | $ 47 |  |  |  |
Interest rate swap | Cash flow hedges | Interest rate risk | U.S. Dollars |  |  |  |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |  |  |  |
Notional amount |  |  | $ 150 |  |  |
Gain (loss) on hedge ineffectiveness | 3 |  |  |  |  |
Hedging instrument, liabilities |  |  |  | (3) |  |
Interest rate swap | Cash flow hedges | Interest rate risk | Canada, Dollars |  |  |  |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |  |  |  |
Notional amount |  |  |  | 75 |  |
Gain (loss) on hedge ineffectiveness | 4 |  |  |  |  |
Hedging instrument, liabilities |  |  |  | (4) |  |
Foreign-denominated debt | Net Investment Hedge | Currency risk |  |  |  |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |  |  |  |
Hedging instrument, assets |  |  |  | $ 472 |  | $ 483
Gain (loss) on hedge ineffectiveness | $ 11 | $ 21 |  |  |  |
Foreign-denominated debt | Net Investment Hedge | Currency risk | U.S. Dollars |  |  |  |  |  |
Disclosure of detailed information about hedging instruments [line items] |  |  |  |  |  |
Notional amount |  |  | $ 370 |  | $ 370 |

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